1. Field of the Invention
The present invention relates to patterning devices for use in lithographic apparatus.
2. Related Art
Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device (which is alternatively referred to as a mask or a reticle) generates a circuit pattern to be formed on an individual layer in an IC. This pattern may be transferred onto the target portion (e.g., including part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned. Manufacturing different layers of the IC often requires imaging different patterns on different layers with different reticles. Therefore, reticles must be changed during the lithographic process.
Existing extreme-ultraviolet (EUV) lithographic apparatus incorporate reflective reticles having substrates formed from ultra-low expansion (ULE) glass, a glass-ceramic material having a coefficient of thermal expansion that is substantially zero across a wide range of operating temperatures. The selection of ULE glass as a substrate is based on the coefficient of thermal expansion of the ULE glass and on an ability to polish a surface of the ULE glass to the fine surface requirements necessary for EUV lithographic applications (i.e., surfaces that exhibit very low roughness, that are substantially free of defects, and that are substantially flat).
In general, existing reflective reticles for EUV lithographic apparatus exhibit a reflectance of approximately 70%. Therefore, depending on a pattern to be printed, an existing reflective reticle can absorb between approximately 30% and 100% of the energy of an incident EUV radiation beam. Such absorption can lead to significant heating of the reticle, which can distort the reticle surface, in spite of the relatively-low coefficient of thermal expansion of the ULE glass substrate, and introduce errors in the projected image.
Further, even if a back side of such a reflective reticle were optimally cooled, the absorption of EUV radiation could result in an excessively large thermal gradient across a thickness of a reticle having a ULE glass substrate. Such excessively-large thermal gradients can result from the relatively-low thermal conductivity of the ULE glass substrate, which promotes a relatively high thermal resistance within the ULE glass substrate and hence, within the reticle. One modification to existing reticle designs that would reduces the thermal resistance of the reticle is to thin the ULE glass substrate, and hence thin the reticle. However, this modification can produce extreme, and potentially insurmountable, difficulties in keeping the patterned surface flat. In addition, such a reticle would deviate from accepted industry thickness for EUV reflective reticles (e.g., approximately 6.35 mm±0.10 mm).